ldap_server: Use an array of struct iovec to avoid data_blob_append()

ldap_server: Use an array of struct iovec to avoid data_blob_append()

This avoids a the implicit 256MB limit on LDAP replies (allowing this
to be increased in the future) and means we copy less memory around.

However because we can only have 1024 entries in a struct iovec (on Linux)
we will need to call tstream_writev_queue_send() multiple times.

Calling it in chunks of 1024 seems a reasonable compromise, the
gensec layer will chunk it out smaller if required.

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Gary Lockyer <gary@catalyst.net.nz>